Typical gas turbine engine fuel supply systems include a fuel source, such as a fuel tank, and one or more pumps that draw fuel from the fuel tank and deliver pressurized fuel to the fuel manifolds in the engine combustor via a main supply line. The main supply line may include one or more valves in flow series between the pumps and the fuel manifolds. These valves generally include at least a main metering valve and a pressurizing-and-shutoff valve downstream of the main metering valve. In addition to the main supply line, many fuel supply systems may also include a bypass flow line connected upstream of the metering valve that bypasses a portion of the fuel flowing in the main supply line back to the inlet of the one or more pumps, via a bypass valve. The position of the bypass valve is controlled to maintain a substantially fixed differential pressure across the main metering valve.
Many aircraft include an engine controller, such as a FADEC (Full Authority Digital Engine Controller), to control engine operation and the fuel supply system. Typically, the engine controller receives various input signals from the engine and aircraft, and a thrust setting from the pilot. In response to these input signals, the engine control system may modulate the position of the above-described fuel metering valve to control the fuel flow rate to the engine fuel manifolds to attain and/or maintain a desired thrust, or, in the case of a turbo prop or turbo shaft engine, a desired speed.
Fuel supply and engine control systems, such as the one described above, may experience certain postulated events that may result in certain postulated failure modes, which in turn may result in certain postulated effects. For example, one particular postulated event is a loss of power. To accommodate this postulated event, the engine control system is typically designed such that, in the highly unlikely occurrence of a loss of power, the fuel metering valve “fails fixed.” That is, the fuel metering valve will remain in the position it was in when the postulated loss of power event occurs. As a result, fuel flow to the engine will remain at the flow rate that was commanded with then postulated loss of power event occurs.
Although the above-described “fail fixed” configuration is generally safe and reliable, it is additionally desirable that the pilot be provided with a means to manually manipulate fuel flow to the engine during the loss of power to the metering valve. The present invention addresses at least this need.